1. Field of the Invention
The present invention relates to a process for bonding and electrically connecting microsystems integrated in several distinct substrates.
2. Description of the Related Art
As is known, numerous technological approaches allow manufacturing integrated circuits wherein the electronic circuitry coexists with a sensor element or an actuator (micro-electromechanical device). The traditional approaches envisage the production of the sensors/actuators and circuitry in a same silicon substrate (surface and epitaxial sensors). The most recent approaches envisage, instead, several substrates and the electronic circuit, the micro-electromechanical device or parts thereof are formed in distinct wafers that are subsequently bonded together and finally diced.
Bonding of the wafers is obtained by causing one or more metals to react with one another, with the silicon of one of the substrates or with metal alloys. To this aim, one or more metals are deposited in sequence on the surface of one or both of the wafers. Then the surfaces to be bonded are brought into intimate contact through a piston that applies a pre-determined pressure, as shown in FIG. 1, which illustrates a substrate 1, a first wafer 2, a layer of bonding material 3, a second wafer 4, and a piston 5 which presses the second wafer 4 against the first wafer 2.
Under the pressure of the piston 4, the bonding material reacts only where the surfaces are in a mechanical contact, and the areas that are not in contact are not bonded.
With this solution, bonding between the wafers depends to a large extent upon the mechanical force of the piston; in particular, criticalities are linked, on the one hand, to the uniformity of pressure applied by the piston and, on the other, to the possible presence of foreign bodies.
In particular, for example in the presence of non-planar areas, the pressure applied by the piston may be non-uniform over the entire surface or over the entire area where bonding is to be obtained. In this case, the presence of areas of the two wafers that are not in contact prevents bonding of these areas.
In addition, the presence of particles, acting as spacers, also entails absence of contact, which prevents bonding, as shown, by way of example, in FIG. 1, wherein a particle 7 prevents bonding in an area of the surfaces of the wafers 2, 4.
On the other hand, application of excessive pressure in an attempt to achieve uniform contact in the areas to be bonded may be counterproductive. In fact the deformation of the substrate thus induced causes stresses in the material that persist over time, weakening the bonding joints and/or subsequently causing undesired deformations, in particular in case of suspended structures. For example, a mobile part (such as a rotor of a micro-actuator), once it is released, tends to relieve the accumulated stresses. In this case, the mobile part may get deformed and undergo an undesired spatial displacement, such as might impair proper operation of the structure or, in any case, reduce efficiency thereof.